The use of a nonvolatile memory in which data is preserved by means of a back-up battery is disclosed in U.S. Pat. No. 4,451,742. The use of a ROM patch in order to correct for errors in a ROM is also known. The ROM patch includes circuitry which recognizes a particular ROM address and disables the old data, substituting corrected data on the output pins. One example of the use of a ROM patch is given in U.S. Pat. No. 4,319,343.
The prior art ROM patch circuits experience difficulty in making the patching process accessible to and convenient for the user--involving the use of dedicated pins in the integrated circuit or the need to interrupt the operation of the system in order to carry out a lengthly alteration process.